Induction imaging system

ABSTRACT

SEQUENTLY APPLYING ELECTROSCOPIC MARKING MATERIAL TO THE RECEIVING MEMBER WHILE IT IS IN CONTACT WITH THE SECOND IMAGING MEMBER OR IMMEDIATELY AFTER SEPARATION THEREFROM THUS SIMULTANEOUSLY FORMING A PLURALITY OF VISIBLE POWDER IMAGES ON THE RECEIVING MEMBER. THE DEVELOPED POWDER IMAGES MAY BE FIXED TO THE RECEIVING MEMBER OR TRANSFERRED TO A FINAL COPY SHEET AND FIXED THEREON.   A METHOD FOR SIMULTANEOUSLY FORMING A PLURALITY OF VISIBLE POWDER IMAGES ON A RECEIVING MEMBER IS DESCRIBED. THE METHOD COMPRISES BRINGING A SUBSTANTIALLY HUMIDITY INTENSITIVE RECEIVING MAMBER HAVING CONTROLLED CONDUCTIVITY CHARACTERISTICS INTO CONTACT WITH A FIRST IMAGING MEMBER BEARING AN ELECTROSTATIC CHARGE PATTERN SO AS TO INDUCE A LATENT ELECTROSTATIC IMAGE IN THE RECEIVING MEMBER, SEPARATING THE LATTER FROM THE FIRST IMAGING MEMBER WHILE CONTACTING THE RECEIVING MEMBER WITH A SECOND IMAGING MEMBER BEARING AN ELECTROSTATIC CHARGE PATTERN AND SUB-

June 12, 1973 R. w. GUNDLACH 3,

INDUCT ION IMAGING SYSTEM Filed Dec. 21, 1970 INVENTOR. ROBERT W GUNDLACH f, D. mac w/m ATTORNEY United States Patent U.S. Cl. 11717.5 30 Claims ABSTRACT OF THE DISCLOSURE A method for simultaneously forming a plurality of visible powder images on a receiving member is described. The method comprises bringing a substantially humidity insensitive receiving member having controlled conductivity characteristics into contact with a first imaging member bearing an electrostatic charge pattern so as to induce a latent electrostatic image in the receiving member, separating the latter from the first imaging member while contacting the receiving member with a second imaging member bearing an electrostatic charge pattern and subsequently applying electroscopic marking material to the receiving member while it is in contact with the second imaging member or immediately after separation therefrom thus simultaneously forming a plurality of visible powder images on the receiving member. The developed powder images may be fixed to the receiving member or transferred to a final copy sheet and fixed thereon.

BACKGROUND OF THE INVENTION This invention relates in general to visible powder im ages such as are useful in xerographic reproduction and, in particular, to a method for simultaneously forming a plurality of visible powder images on a surface of a receiving member.

In the practice of xerography it is the general procedure to form a latent electrostatic image on a xerographic Plate generally by charging a photoconductive insulating surface and dissipating the charge selectively in accordance with a pattern of activating radiation corresponding to an original image. In the ordinary situation with information to be reproduced the charged plate is exposed to a single original image and developed with an electroscopic marking material to form a visible powder image thereon which is subsequently transferred to a recording medium and fixed thereto, the cycle being repeated any number of times to produce any desired number of reproductions of the original image. New developments in information reproduction have now made it advantageous to copy more than one original image on a recording medium such as where computer-generated information is intended to be copied in conjunction with special forms. This has been accomplished heretofore by using overlay transparencies corresponding to the forms together with the original information sought to be inserted thereon and exposing a charged xerographic plate to the combination. Highly satisfactory results are obtained according to this procedure; however, the transparency must initially be made prior to reproduction of the information on the forms thus causing delay and added expense. There are also problems with respect to maintaining intimate contact of the overlay with the opaque reproduction subject during exposure which can lead to shadow effects, i.e., double lines in the finished copy.

It is known to form developed powder images by means of an induction imaging technique wherein a receiving member is brought into contact or virtual contact with a surface bearing a latent electrostatic image resulting in the formation of an induced latent electrostatic image in the receiving member which can then be developed by the application of electroscopic marking materials. Such a method is described in detail in copending application Ser. No. 867,049 filed Oct. 16, 1969 now U.S. Pat. 3,551,146. Induction imaging provides numerous advantages among which is that many reproductions of an original image may be made from only a single electrostatic latent image. Thus, the necessity for forming a latent electrostatic image on an imaging member for each reproduction desired is avoided. Furthermore, it is possible to practice induction imaging in a manner such that the process is substantially insensitive to ambient humidity conditions; wherein any material may be utilized as a final copy support member and also wherein embodiments which are particularly adapted to the reproduction of line copy only or to solid area development as well as line copy development may conveniently be selected. Such techniques are described in detail in copending application Ser. No. 50,- 763, filed June 29', 1970 now U.S. Pat. 3,703,376.

So called interposition development has been disclosed in U.S. Pat. 2,297,691. This system obviates abrasion damage to an imaging member, such as, a xerographic plate since a receiving member, such as paper, is placed in contact with the plate having a latent electrm static image thereon and toner is applied to the back of the receiving sheet and fixed thereon. Thus, no contact of abrasive toner or carrier beads with the imaging member is required and no cleaning of residual toner from the imaging member is necessary. Moreover, interposition development requires only a very short contact time between the imaging member and the copy receiving member in order to produce highly satisfactory copies.

It would be desirable to have a method for copying a plurality of original images in combination on a receiving member wherein the visible images are simultaneously formed directly from the original images sought to be reproduced without the necessity of initially having to make any overlay transparencies and even more so to have such a method which encompasses the above-described advantages.

SUMMARY OF THE INVENTION It is an object of this invention to provide a novel imaging method which overcomes the above mentioned difficulties of the prior art and provides the above-described desirable features.

It is another object of this invention to provide an imaging method wherein a plurality of visible powder images are simultaneously formed on a receiving member.

It is still another object of the invention to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member wherein two imaging members, each carrying an electrostatic charge pattern on a surface thereof are employed and wherein development of electrostatic latent images is not carried out on the surfaces on which the original electrostatic charge patterns are formed.

It is a further object of the invention to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member wherein a plurality of electrostatic latent images are found in the receiving member by induction imaging techniques.

It is a still further object of the invention to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member wherein induction imaging techniques are combined with interposition development techniques.

It is another object of the invention to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member which is capable of being practiced in a continuous mode.

Yet still another object of the invention is to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member which 18 substantially h'umidity insensitive.

It s a further object of the invention to provide an imaging method for simultaneously forming a plurality of visible images wherein the final copy sheet may be any material. I

Still another object of the invention is to provide an imaging method for simultaneously forming a plurality of visible images on a receiving member wherein the original images sought to be reproduced may be either positive or negative in image sense.

The foregoing objects and advantages are realized according to the invention by providing a receiving member in the form of a sheet or a web, preferably comprised of substantially humidity insensitive material and having controlled electrical conductivity characteristics which is brought into successive contact or virtual contact with a plurality of imaging members carrying electrostatic charge patterns or latent images. The receiving member is brought into contact with a first imaging member carrying an electrostatic charge pattern in a manner such as to induce a latent electrostatic image in the receiving member, then separated from the first imaging member and brought into contact with a second imaging member carrying an electrostatic charge pattern. Subsequently, electroscopic marking material is applied to the receiving member while it is in contact with the second imaging member or immediately after separation therefrom. In this manner a plurality of visible powder images may be formed simultaneously on the receiving member. According to one embodiment of the invention a plurality of electrostatic latent images are induced in the receiving member Web by maintaining the receiving member in contact with the second imaging member for a period of time sufficient to induce in the web a latent electrostatic image corresponding to the electrostatic charge pattern carried by the second imaging member. It should be recognized that, in the practice of this embodiment, while the second electrostatic image is being induced in the receving member web the first electrostatic image induced in the Web through its contact with the first imaging member will lose some of its density and resolution. Therefore for best results in any instance it is preferred to initially induce the least critical image, i.e., the one for which less density and resolution in the corresponding developed visible image can be tolerated, and to subsequently induce the more critical image in the Web. For example, where computer-generated variable information is being reproduced in conjunction with a special form the latent image corresponding to the latter is preferably induced in the web first followed by that corresponding to the variable information. Upon separation of the receiving member from the second imaging member, the application of electroscopic marking material to the receiving member will result in the simultaneous formation thereon of visible powder images corresponding to the electrostatic charge patterns carried by the first and second imaging members respectively.

In accordance with another embodiment of the invention the receiving member is brought into contact with the second imaging member for only a very short period of time and the electroscopic marking material is applied to the receiving member while it is in contact with the imaging member. By means of this technique the electrostatic fields associated with the electrostatic charge pattern carried by the second imaging member can be developed on the receiving web by interposition development simultaneously with the development of the induced latent image in the Web corresponding to the electrostatic charge pattern carried by the first imaging member. The developed images can be fixed to the receiving member or transferred to a final copy sheet and fixed thereto.

The imaging method of the invention provides a convenient and very useful technique for combining reproduction of original images on a single copy sheet. The imaging member need not be cleaned of any residual electroscopic marking material since none is ever brought into direct contact with the surfaces thereof. By providing a receiving member which is in the form of a continuous recycling web the method can be practiced in a continuous mode and, furthermore, Where a plurality of reproductions is desired or where variable information is desired to be reproduced in conjunction with special forms the method offers a convenient means whereby a latent electrostatic image need not be formed for each reproduction. By utilizing substantially humidity insensitive materials as the receiving member according to a preferred embodiment of the invention the method may be practiced substantially without regard to environmental conditions. The developed images may be transferred to a final copy sheet comprising substantially any material and, within practical limits, any thickness thus making the method substantially independent of the properties of the final copy sheet. It will be readily seen that the method of the invention offers numerous advantages and is extremely versatile in that particular embodiments may be selected with respect to certain variables such as the type of original images sought to be reproduced as will be discussed in detail further below.

The foregoing and other objects and advantages of the invention will be more fully understood from the following detailed description of the preferred embodiments thereof, particularly, when read in relation to the drawings in which:

FIG. 1 is a partially schematic illustration of the practice of the imaging method of the invention according to a preferred embodiment thereof;

FIG. 2 is a graphical illustration showing the condition of the receiver web with respect to the charge density of the laten image induced therein according to one embodiment of the invention; and

FIG. 3 is a partially schematic illustration of the practice of the imaging method of the invention according to a preferred embodiment thereof.

Referring now to FIG. 1 there is seen a receiving member 10 having controlled electrical conductivity characteristics which generally comprises moisture insensitive material and which may be a sheet of material or preferably a continuous recycling web. The receiving member 10 can be any suitable self-supporting film member, which may be prepared from a wide variety of materials. It is generally preferred that the film material be slightly conductive having a bulk resistivity ranging from about l0" to about 10 ohm-cm. Within this range, it is preferred that the resistivity of the film be about 10 to about 10 ohm-cm. and that the web material be moisture resistant through all practical humidities so as to permit optimum use under all environmental conditions. Although this latter requirement is not absolutely necessary, it is desirable to lend the necessary versatility and flexibility to the proposed system and permit extensive use as required. Thus, this particular embodiment of the system may be utilized, if desirable, under environmental conditions of relative humidity readings greater than 75 percent. The induction film material shall be durable enough to withstand the mechanical stresses of continued recycling and as thin as possible to insure and maintain the highest degree of image resolution with thickness ranging from about 0.5 to about 3 mils with a preferred range for optimum results being from about 1 to about 1.5 mils. Typical materials capable of satisfying the above requirements include: doped Mylar, polypropylene, polyethylene, Tedlar, and various resin impregnated papers such as polyethylene impregnated paper or pigment filled papers. As mentioned above, the web material or film is generally preferred to be slightly conductive so that the electrostatic latent image is induced in rather than on the surface of a film. In addition, the relative conductive arsasss nature of the permanent induction imaging Web leads to a dissipation of the image induced in the film thereby permitting recycling without accumulating residual images.

Receiving member is brought into contact with a first imaging member, generally designated 12, carrying an electrostatic charge pattern or latent image 14. The electrostatic charge pattern or latent image can be formed by any one of a number of suitable techniques. For example, an electrostatic latent image may be formed on a photoconductive insulating surface through the steps of charging the surface and selectively dissipating the charge by exposure to a pattern of electromagnetic radiation according to the process disclosed in US. Pat. No. 2,297,- 691. Other methods of forming the electrostatic charge pattern may be used, such as, for example, by selective deposition of electrostatic charge, as by impressing a charge through an image stencil on an insulating surface to form a pattern of such charge, imposing a potential on a shaped conductor or electrode, or the like. Thus, the image may be formed on a xerographic photosensitive member comprising a photoconductive insulating layer overlying a conductive substrate or on such other combination as may be desirable to provide an electrostatic charge pattern on an insulating surface.

The imaging member which bears the electrostatic charge pattern may be of any suitable material capable of holding electrostatic charge for suflicient time to permit the desired number of copies to be produced. It is desirable that the particular image bearing material be, by nature, flexible so as to conform to the curvature of a rotating drum upon which it is attached according to a preferred mode of the invention. Typical such materials are Mylar, polyethylene terephthalate, commercially available from -E. I. du Pont de Nemours, Inc.; Teflon, polytetrafluorethylene, commercially available from I. du Pont de Nemours, Inc.; Tedlar, polyvinylfluoride, commercially available from E. I. du Pont de Nemours, Inc.; Staybelite resins, a family of thermoplastic synthetic resins prepared from hydrogenated resin, commercially available from the Hercules Powder Company, styrene polymers such as Velsicol, a styrene terpolymer, commercially available from the Vesicol Chemical Corporation; and the Piccolastic resins, a styrene polymer available from the Pennsylvania Industrial Chemical Corporation, ethyl cellulose; cellulose acetate; polycarbonates such as Plestar, commercially available from General Aniline and Film Company; polyethylene; polypropylene; polymeric materials such as casein and 'Parlon-P, the latter being a chlorinated natural rubber commercially available from the Hercules Powder Company; and polyvinyl chloride. When photosensitive plates are utilized or the rotating drum itself is photoconductive typical materials which may be used include the conventional photoconductive insulating material such as of the nature disclosed in US. Pat. Nos. 3,121,006 and 3,121,007. When the photosensitive material is utilized the base or backing substrate used in preparing the respective photoconductive plate is generally chosen so as to satisfy the desired flexibility requirements of the present system. Generally, therefore, most of the conventional support materials maye be used such as aluminum, brass, copper, zinc, paper and any suitable plastic substrate or the like having the necessary conductivity properties.

When the electrostatic latent image or charge pattern is formed by way of a photoconductive insulating plate the surface of the plate is uniformly charged as by corona discharge in the dark and the surface selectively exposed to an electromagnetic radiation source. Due to the photoconductivity of the layer the charge will be dissipated in those areas which are struck by light. Typical photoconductive materials which are suitable for use in the corresponding photoconductive insulating layer are selenium, sulfur, anthracene, inorganic photoconductive pigments such as zinc oxide or cadmium sulfide dispersed in inert binder resins, organic photoconductive pigments such as phthalocyanine, dispersed in inert binder resins, a homogeneous photoconductive layer or organic photoconductive material, such as, for example, poly-N-vinyl carbazole sensitized with 10% by weight of 2,4,7-trinitro-9- fluorenone and charge transfer complexes of Lewis acids and aromatic resins such as are disclosed in copending US. patent application Ser. No. 426,409, filed Jan. 18, 1965 now US. Pat. 3,408,183, and having a common assignee.

In the present illustration the image support member 12 is a photoconductive insulating layer 16 overlying a conductive substrate in the form of a rotating drum 18. Photoconductive insulating layer 16 carries on its surface an electrostatic charge pattern 14 representing the pattern to be reproduced. In this exemplary instance the electrostatic charge pattern 14 is formed by uniformly electrostatically charging imaging member 12 at a corona charging station, generally designated 20 and subsequently exposing the charged imaging member to a pattern of electromagnetic radiation at an exposure station generally designated 21 which in this exemplary instance is made up of a light source 22, a transparency 23 and a lens 25.

The image support member 12 is then disposed in contact or virtual contact with the inner surface of receiving member 10 separated by only a very thin air gap. According to one embodiment of the invention, there is positioned in close proximity to the web an optional charging station or corona grounding station generally designated 24. While the image support member is in contact with the receiving member 10 a potential can be applied to the free outer surface of the continuous film by corona discharge station 24. As a result of this applied potential an enhanced electrostatic latent image is induced in the first or inner surface of the continuous film corresponding to the electrostatic charge pattern 14 on the image support member 12. Depending upon the polarity of the electrostatic charge of the latent image, the potential applied at the charging station 24 to the outer or second surface of the web may be negative, positive, ground or of an alternating nature. The potential applied to the free outer surface of the continuous film (functions as a reference potential to give development of solid area images. Thus, with respect to the reproduction of original images which have wide, solidly dark areas as Well as, or in addition to, line copy it is preferred to employ the technique of applying a potential to the continuous web in the manner described. It will be appreciated by those skilled in the art that the applied potential should preferably be closely matched to the potential of the background or non-image areas to avoid high background or washed out low density images. An induced image will be realized even if this corona grounding device is eliminated, however, with an edgeonly characteristic which lacks solid area reproduction. In this latter instance the continuous web must be slightly conductive.

Externa grounding of the free surface of the receiving member is preferred in many imaging applications where solid area development is desired; however, especially for line copy reproduction which comprises the bulk of ofiice copying, a xerographic development system which senses only differences in potentials instead of absolute potentials is suitable to provide a development system which yields copies having clean backgrounds over a wide range of image exposures. Accordingly, in another preferred embodiment of the method, the corona grounding device previously described is eliminated and the receiving member 10 is not contacted with any externally applied potential while in contact with the image support member 12. With respect to this mode of the induction imaging system, an image is again induced in the receiving member 10; however, the induced image has an edgeonly characteristic which lacks solid area development with the result that solid areas, considerably over about inch in dimension, show edge-only or edge enhanced development which is acceptable in line copy imaging requirements found, for example, in most office documents. In this latter embodiment of the method the electric fields in the continuous web are located on both sides of a boundary of about inch in dimension and it will therefore be apparent that areas about inch in dimension will be completely developed.

The receiving member web 10 hearing the induced image is then separated from imaging member 12 while contacting a second imaging member, generally designated 28, illustrated in the form of an endless recirculating belt. The second image support member 28 may be constructed in accordance with any of the configurations and from any of the materials described above with relation to the first image support member. As illustrated image support member 28 is a thin continuous film of photoconductive insulating material 30 wrapped around drive rollers 31 and 31'. An electrostatic charge pattern 32 is formed on the photoconductive insulating film 30 by uniformly electrostatically charging the film by corona charge station 34 and subsequently exposing the charged film to imagewise actinic radiation at exposure station 36. The receiving member 10 is held in pressure contact with both the first image support member 12 and the second image support member 28 simultaneously. For the relatively short contact time the image field associated with the electrostatic charge pattern 32 on the second image support member 28 can be developed on the interposed receiving member 10. At the same time the latent image induced in the receiving member by the first image support member 12 is also developed thereby simultaneously forming a plurality of visible images 38 on the receiving member.

In order to achieve optimum image density and resolution the time that the receiving member is positioned in virtual contact with electrostatic charge pattern on the first image support member should be at least three times longer than the time between separation of the receiving member from the image support member and development of the induced image. Because of this consideration, it is preferred in the embodiment of the invention illustrated in FIG. 1 to provide the second image support member, generally designated 28, in the form of a recycling belt. Thus, since interposition development can be conveniently accomplished only with a very short contact time, employing a recycling web as the second image support member permits sharp visible images to be produced since the induced image in the receiving member is developed within the relaxation time of the receiving member material. However, the induced latent image may be developed at any time before it decays, i.e., is substantially completely dissipated, but with slightly poorer density and resolution. Detailed discussions relating to considerations of induction time and relaxation time for any receiving member are found in co-pending application Ser. No. 867,049 tfiled Oct. 16, 1969 now US. Pat. 3,551,- 146 and copending application Ser. No. 50,763 filed June 29, 1970, both having a common assignee with the present invention and both of which are herein incorporated by reference.

The visible powder images 38 can be formed on the continuous web by any one of many suitable development techniques. Because of the time considerations involved it is preferred to employ a confined band developer system, i.e., one in which electroscopic marking material can be presented to a relatively small area of the latent images to be developed and then quickly removed so that at any given point in time only a small segment of the overall image area is being developed. The development system, generally disignated 40, is preferably a magnetic brush developer as shown in the present illustration. The magnetic brush consist of a magnet on the surface of which is held a magnetic carrier. The magnetic field holds the carrier particles in a brush-like configuration. As this brush passes over 1? areas encompassing the induced latent image and the electrostatic fields associated with the electrostatic charge pattern on the second imaging member, which fields extend through the receiving member web while it is in contact with the second imaging member, electrostatically charged toner particles are attracted to the outer surface of the web corresponding to the induced latent image in the web and the electrostatic charge pattern on the second imaging member. Such magnetic brush development is described in more complete detail in US. Pats. 2,975,758 and 2,846,333. Other suitable developer methods which may be employed include electroded powder cloud development as described in US. Pat. 2,221,776, touchdown or transfer development from a toned donor surface as is described in U.S. Pat. 2,895,847, electrophoretic development as is described in U.S. Pat. 3,068,- or, generally speaking, any development method in which development is accomplished over a relatively short space and preferably, in some cases, employing a development electrode to prevent sparking of the latent images.

The developed images can then be fixed to the receiving member by any suitable image fixing technique such as heat or vapor fusing or preferably transferred to a permanent copy support by any suitable image transfer technique such as by electrostatic means and fixed thereto. The latter embodiment is preferred since the receiving member web may then be cleaned of any residual toner and recycled for subsequent use thus permitting the method to be practiced in a continuous mode. The permanent copy support may consist of conventional opaque moisture absorbing or humidity sensitive or humidity insensitive material of any desired thickness. Typical such materials include ordinary bond paper and resinous transfer materials such as Mylar, polypropylene, polyethylene, Tedlar and the like.

The reproduced copy of the original images preferably is comprised of a plurality of visible images which are positive in image sense, i.e., they consist of dark or black image areas on a white or opaque background. In order to achieve such a result according to the embodiment previously described either the image sense of the respective electrostatic charge patterns on the first and second imaging members must be opposite or, alternatively, the charge polarity thereof must be opposite. For example, the electrostatic charge pattern carried by the first imaging member may be positive in image sense, i.e., where the image areas hear an electrostatic charge and are surrounded by relatively discharged background areas, and comprised of electrostatic charges of positive polarity while that carried by the second imaging member is positive in image sense but comprised of electrostatic charges of negative polarity. Alternatively, the electrostatic charge pattern carried by the first imaging member could be positive in image sense and comprised of electrostatic charges of positive polarity while the charge pattern on the second imaging member is negative in image sense, i.e., where the image areas are relatively discharged and are surrounded by charged background areas, but comprised of electrostatic charges of positive polarity, etc. Induction imaging is capable of negative to positive processing as is interposition development, the latter development technique permitting such processing when a biased developer system is used in which case the developer is preferably biased to a potential matching the (charged) background areas of the latent image. In this manner, it will be recognized that visible images which are positive in image sense are simultaneously formed when electroscopic marking material is applied to the surface of the receiving member web. The charge polarity of the electroscopic marking material which must be employed to achieve the plurality of images which are positive in image sense will be dependent upon the charge polarity of the respective electrostatic charge patterns on the respective imaging members and the image sense of the charge patterns. In any event, it is possible to select a combination which will allow all the visible powder images to be formed simultaneously by the application of electroscopic marking material which has a uniform polarity of charge.

With respect to induction imaging in general it will be recognized that the condition of the charge pattern induced in the receiving web is dependent upon whether the web is externally grounded while in contact with the imaging member. When the external grounding mode is practiced the charges induced in the web will, generally speaking, be opposite in polarity to those in corresponding areas of the original (or master) latent image. When the edge-only mode is employed, i.e., Where the web is not externally grounded, a somewhat different situation exists. In this case the induced charge pattern is comprised of negative charges and positive charges. It will be understood by those skilled in the art that the number of positive and negative charges induced in the receiving Web in this case are exactly equal thus resulting in a net induced charge of zero. Wherever a latent image is induced in the receiver sheet, electric fields arrange themselves on both sides of a boundary, typically about inch in dimension and within which dimension line copy (such as the characters on this page) is generally encompassed. This phenomenon is graphically illustrated in FIG. 2. Consider the situation where a photoconductor is charged to a potential of +800 volts and then selectively discharged by contacting it with a light and shadow pattern corresponding to an original image. The areas struck by the incident light will be discharged but not completely. For purposes of illustration the discharged areas can be considered as having a +300 volt potential. The areas not struck by light, on the other hand, will remain at a potential of substantially +800 volts. When the charge pattern is brought into contact with the receiving web the following condition will occur in the areas of the web corresponding to those areas of the master latent image where a boundary exists, i.e., where areas at +800 volt potential are adjacent to areas at +300 Volt potential: the boundary portions of the master latent image at the higher potential will induce negative charges in the corresponding areas of the web; the boundary portions of the master latent image at the lower potential will induce positive charges in the corresponding web areas since the master latent image areas at lesser potential are negative with respect to the areas thereof having the higher potential. Thus in FIG. 2 there is graphically represented the charge density for the latent image induced in the web. This is intended to be illustrative of the condition which exists on both sides of any boundary about inch in dimension. Areas P1 and P2 represent the charge density of the negatively charged portions and N1 and N2 represent the charge density of the positively charged portions. Of course as has been discussed P1 and P2 are exactly equal to N1 and N2.

In the case where computer-generated information is being reproduced on special forms, the variable information from the computer will typically be provided as a cathode ray tube (CRT) projection which is negative in image sense. Since induction imaging preserves the master electrostatic charge pattern through at least 100 duplicates it is preferred to form a latent electrostatic image which is positive in image sense on the first image support member and which corresponds to the special form and to project the variable information on the other image support member in the form of a CRT projection. Thus, according to the embodiment described, by forming such electrostatic charge patterns having the same polarity of charge on the respective imaging members and utilizing a biased conductive developer system, visible images which are positive in image sense and which correspond to the special form an variable information are conveniently produced on the continuous web.

According to another preferred embodiment of the invention, a plurality of latent electrostatic images may be induced substantially simultaneously or sequentially in the receiving member by bringing the continuous web into contact or virtual contact with two imaging members successively. This embodiment is shown in FIG. 3 which uses identical numerals to identify parts of the device which are identical with those shown in FIG. 1. This particular embodiment differs from that described extensively above in that the receiving member web 10 is held in contact with the second imaging member for a period of time sufiicient to induce a latent image in the Web corresponding to the electrostatic charge pattern carried by the imaging member; and further in that development of the induced latent images in the web is carried out after the web is separated from the second imaging member.

The second imaging member 42 employed according to this embodiment is preferably, as illustrated, a recycling flexible conductive belt 44 carrying a photoconductive insulating layer 46 bearing an electrostatic charge pattern 48.

This configuration is preferred for the second imaging member in this instance since it allows the receiving member Web to be held in contact therewith for a period of time sufficient to induce, in the receiving web, a latent image corresponding to the electrostatic charge pattern carried by the imaging member. Thus, according to this embodiment, a plurality of latent images corresponding to the respective electrostatic charge patterns carried by the first and second imaging members are formed in the web and developed by the application of electroscopic marking material immediately upon separation of the Web from the second imaging member. The induction time, i.e., the time for which the receiving member must be in contact with the imaging member in order to induce a latent image in the web is dependent upon the bulk resistivity of the receiving member material, the thickness thereof and the spacing of the air gap between the imaging member and the receiving member surface. The induction time for typical receiving members having resistivity of between about 10" to about 10 ohm-cm. can vary from about 10* seconds to about seconds. Furthermore, for optimum results the induction time should be at least three times longer than the time between stripping the web from the particular imaging member and development.

Because of the foregoing considerations it is preferred to induce both sets of latent images in the web substantially simultaneously. This is accomplished as shown by bringing the receiving web into contact with the second imaging member shortly after the web is contacted by the first imaging member. In this manner optimum image density and resolution for the developed images obtained from the latent images induced in the web by both imaging members are obtained. However it should be apparent that acceptable visible images may be obtained when the respective latent images carried by the imaging members are induced sequentially in the receiving Web, i.e., Where the second imaging member is brought into contact with the web after the Web has been in contact with the first imaging member for a relatively longer period of time. According to this embodiment it will be apparent that some difficulty may be encountered with respect to achieving image density and resolution for the developed image obtained from the latent image induced in the web by the first imaging member. Since the continuous web must remain in contact with the second imaging member for a period of time sufiicient to induce a latent image in the web, the first induced latent image may decay some what by the time development of the induced latent image is carried out thus resulting in a slightly fuzzy visible image corresponding to the first induced latent image. This effect may be compensated for by various techniques. For example, the receiving member web may be held in contact with the first imaging member for a longer period of time. Free surface developmnt, as illustrated in FIG. 3, can aid in overcoming such undesired efi'ects since access time to the induced latent image is relatively short after separation of the web from the second imaging member and thus is the preferred development mode. However, it should be understood that development may be carried out on the other surface of the web. The latter mode is more sensitive to humidity since access time is relatively longer compared with the former method in which development takes place simultaneously with the generation of electrostatic fields in the web upon separation of the web from the imaging member and the developed images would suffer some loss in image density and resolution.

Relative to the embodiments of the invention wherein the substantially simultaneous or the sequential induction modes are practiced it should be recognized that, in some cases, the developer process employed must be a conductive system which may be ungrounded or at a bias in the order of between ground and not to exceed the bias which would degrade the induced latent image. By conductive developer is meant a development system which includes at least one conductive element. The conductive developer system must be used in conjunction with the substantially simultaneous or the sequential induction mode when an external potential is applied to the surface of the receiving member while the latter is in contact with the first imaging member. It is necessary in order to prevent ionization of the first induced image at the point of separation of the receiving web from the second imaging member. In addition, the conductive developer assists in the complete elimination of any residual image in the web. Preferably, for best quality images, the conductive developer should be held at a bias which is about the same as the potential applied to the web while it is in contact with the first imaging member. A magnetic brush development system is preferred because it is the most expedient manner of presenting a conductive developer to the web surface.

In the mode where the continuous receiving member web does not receive a potential from external means while in contact with the first imaging member, the development technique utilized need not be a conductive system. According to this mode where edge-only development occurs the electric fields in the continuous web are located on both sides of a boundary of about inch in dimension and do not extend to the imaging member during separation of the web therefrom. Thus, when separation takes place, no ionization of the induced image occurs which could produce a field sufficient to cause field breakdown and result in destruction of the latent image carried by the imaging member as well as the induced latent image in the web. Therefore any development system which responds to a given image polarity may be employed.

The invention will now be further described With reference to specific preferred embodiments thereof in the form of examples, it being understood that these are intended to be illustrative only and the invention is not limited to the specific materials and conditions recited therein. All parts and percentages stated are by Weight unless otherwise indicated.

EXAMPLE I A first imaging member comprising an aluminum substrate in the form of a rotary drum having a 50 micron layer of vitreous selenium coated thereon is uniformly positively charged to a potential of about 800 volts by means of corona discharge in the dark. A wrong-reading light-and-shadow pattern corresponding to a form on which data is to be inserted is projected onto the plate by means of a lens, thereby dissipating the charge in the light struck areas and creating a latent electrostatic image thereon which is positive in image sense, that is, charged image and discharged background areas. A receiving member comprising a sheet of Tedlar is placed in contact with the first imaging member and maintained in contact for a period of time sufiicient to induce a latent image in the sheet. The receiving member is then separated from the first imaging member and brought into contact with a second imaging member bearing an electrostatic charge pattern. The second imaging member comprises a thin continuous film of poly-N-vinyl carbazole sensitized with about 10% by weight of 2,4,7-trinitro-9-fiuorenone in the form of a recirculating belt arranged on a pair of drive rollers. The electrostatic charge pattern is formed on the second imaging member by uniformly positively charging the member to a potential of 600 volts and projecting thereon a CRT display corresponding to data desired to be reproduced in conjunction with the special form. Thus, a right-reading latent image which is negative in image sense (discharged image and charged background areas) is created on the imaging member. As the Tedlar sheet is brought into contact with the second imaging member electroscopic marking material is applied to the free surface of the sheet by means of a magnetic brush developer system held at a potential of about 500 volts. The marking material is selected so as to have a negative polarity of charge and the particles are deposited on the Tedlar sheet in a pattern corresponding to the induced latent image on the sheet and also to the electrostatic fields associated with the electrostatic charge pattern carried by the second imaging member, which fields extend through the Tedlar sheet while it is in contact with the second imaging member. The Tedlar sheet bearing the developed images is heated to the fusing point of the toner and then cooled thereby permanently fixing the images. An excellent reproduction of the data on the form is obtained, the images being of high density and resolution. The abovedescribed process is repeated times with 100 sheets of Tedlar. The same electrostatic charge pattern on the first imaging member is utilized for all the additional reproductions .while a new latent image corresponding to different data is formed on the second imaging member for each reproduction. The quality of the images on these sheets continues to be excellent with only a very slight, almost unnoticeable decrease in image density being observed in the special form.

EXAMPLE II The process as described in Example I is repeated except that the Tedlar is in the form of a continuous recycling web and the powder images developed are transferred to a paper copy sheet and fixed thereon. The Tedlar web is cleaned of any residual toner particles and each individual reproduction is formed thereon and transferred to a paper copy sheet.

EXAMPLE III The process as described in Example I is repeated except that the second imaging member is a layer of cadmium sulfoselenide arrayed on a steel substrate. This second photoconductor is negatively charged, brought into adjacency with the first imaging member and maintained in adjacency with the rotary drum by means of two rollers while the Tedlar web is maintained in contact between them for a period of time suflicient to induce two latent images in the web substantially simultaneously corresponding to the electrostatic charge patterns carried by the first and second imaging members. During separation of the Tedlar web from the second imaging member the free surface of the web at the point of separation is in contact with a magnetic brush developer held at a bias of 400 volts. Toner particles which are selected to have a negative polarity of charge are deposited on the Tedlar web in a pattern conforming to the original images. The developed images are transferred to a paper copy sheet and fixed thereon. The Tedlar Web is cleaned of any residual toner particles and the process is repeated 20 times to make 20 duplicate copies using the same latent images on the imaging members. The quality of the images is again excellent with only a very slight decrease in the density of the additional duplicate copies being observed. The lower number of duplicates achievable in this mode reflects the higher dark conductivity of the belt photoconductor, compared with the ability of selenium to hold charge over long periods of time.

EXAMPLE IV The process as described in Example III is repeated except that a wrong-reading image representing a form is projected from a microfilm which has bright characters on a dark background thereby creating a latent image on the selenium drum which is negative in image sense, i.e., it has discharged image areas surrounded by charged background areas. A second photoconductor comprising a 50 micron thick layer of vitreous selenium on a 6 mil thick brass substrate is positively charged by corona means and preferentially discharged in a right-reading ima e pattern by exposure to a CRT image display. This flexible second photoconductor is then brought into adjacency with the first photoconductor over sutficient arc to induce electrostatic latent images in a Tedlar Web held between the photoconductors. After induction of the two sets of electrostatic latent images in the Tedlar web it is separated from the first photoconductor and subsequently, immediately as the web is separated from the second photoconductor the latent images in the web are developed with a magnetic brush containing negatively charged toner particles. The same latent images are retained while the two photoconductors are recycled 100 times to make 100 duplicate copies. The copies are all of excellent quality with only a very silght decrease in the density of the additional copies being observed.

. EXAMPLE V The process as described in Example I is repeated with the exception that S. D. Warren thintext paper is used as the receiving member and the process is carried out in an environment having a relative humidity between 20% and 50% RH. The quality of the images on these sheets is comparable with those obtained in Example I.

EXAMPLE VI The process as described in Example I is repeated. The first imaging member is exposed to an original image comprising solid image area-s. During the time the first imaging member is in contact with the Tedlar sheet, the latter is contacted with a potential of about +300 volts by means of corona discharge in order to induce charge over solid areas in the sheet. The latent image on the second imaging member is negative in image sense. As the Tedlar sheet is brought into contact with the second imaging member electroscopic marking material having a negative polarity of charge is applied to the free surface of the sheet by means of a magnetic brush developer system held at a bias of about +500 volts.

Although the present examples are specific in terms of the conditions and materials used, any of the typical materials may be substituted when suitable and the examples with similar results being obtained. In addition, other materials may be incorporated in or coated on the developer, photoconductor and receiving member which will enhance, synergize or otherwise desirably aifect the properties of these materials for their present use. Furthermore, variations of the method may be conveniently practiced. For example, the electrostatic charge pattern carried by either of the imaging members may be comprised of more than one latent electrostatic image. This may be accomplished by utilizing an imaging member which comprises a xerographic plate having adhered to the photoconductive surface thereof an electrically insulating film. A method for forming a plurality of latent electrostatic images on such an imaging member is disclosed in co- 14 pending application Ser. No. 50,216, filed June 26, 1970 now abandoned and replaced by continuation-in-part application Ser. No. 174,380, filed Aug. 25, 1971.

It will be appreciated by those skilled in the art that the method of the invention is an extremely valuable technique since, within practical limits, it is in most cases not dependent upon environmental conditions nor on the characteristics of the final copy sheet thus permitting wide latitude in the selection of materials and placing substantially no restrictions on the conditions under which it is carried out. As has been described, particular embodiments of the method may be conveniently chosen with respect to the type or original images desired to be reproduced in combination. Although the method is intended to be practiced in a manner such that registration of the respective images on the receiving member web will preclude any overlap in the reproduced images it should be recognized that some overlap may occur without substantially affecting the results obtained. With respect to the reproduction of line copy which typically comprises the bulk of the copy reproduced, substantial areas of the original material are comprised of background. For example, a US. patent, on pages completely filled with single space information, has a background area in the order of about to with about 5% to about 10% of the total imaging area of the page taken up by character information. Thus, it will be clearly evident that any overlap, between, for example, character information and a standard form would not present any significant problems in the practice of the invention. Further, although the method has been described with relation to the combination of information or certain forms it may be employed to reproduce many other combinations.

While the invention has been described in detail with respect to various preferred embodiments thereof and further in relation to specific examples it is not intended to be limited thereto but rather it will be appreciated by those skilled in the art that variations and modifications are possible which are within the spirit of the invention and the scope of the claims.

What is claimed is:

1. A method for forming a plurality of visible powder images on a receiver material comprising the steps of:

(a) providing first and second image support members, each of said image support members bearing an electrostatic charge pattern on a surface thereof;

(b) contacting the electrostatic charge pattern bearing surface of said first image support member with a receiving member having a bulk resistivity of from about 10' to 10 ohm-cm., thereby inducing a latent electrostatic image in said receiving member;

(0) separating said receiving member from said first image support member While contacting the electrostatic charge pattern bearing surface of said second image support member with the surface of said receiving member opposite from that contacted by said first image support member; and

(d) applying electroscopic marking material to said receiving member thereby simultaneously forming a plurality of visible powder images on the receiving member.

2. The method as defined in claim 1 and further including the step of fixing said plurality of visible powder images to said receiving member.

3. The method as defined in claim 1 wherein said receiving member is a continuous web and further including the steps of transferring said plurality of visible powder images from said receiving member to a permanent copy support and fixing said images thereto.

4. The method as defined in claim 3 wherein said receiving member comprises a material having a bulk resistivity of from about 10 to 10 ohm-cm.

5. The method as defined in claim 4 wherein said elec- 15 troscopic marking material is applied by a conductive magnetic brush developer system.

6. The method as defined in claim 1 wherein a potential is applied to said receiving member at least in the portions thereof desired to be imaged, said potential being applied subsequent to contacting said receiving member with said first image support member but prior to separation of said receiving member therefrom and wherein said electroscopic marking material is applied by a conductive developer system.

7. The method as defined in claim 6 wherein said conductive development system comprises a conductive magnetic brush development system.

8. The method as defined in claim 1 wherein said receiving member comprises a substantially humidity insensitive material.

9. A method for forming a plurality of visible powder images on a receiving material comprising the steps of:

(a) providing first and second image support members, each of said image support members bearing an electrostatic charge pattern on a surface thereof;

(b) contacting the electrostatic charge pattern bearing surface of said first image support member with a receiving member having a bulk resistivity of from about to about 10 ohm-cm., thereby inducing a latent electrostatic image in said receiving member; (c) contacting the electrostatic charge pattern bearing surface of said second image support member with the surface of said receiving member opposite from that contacted by said first image support member;

(d) separating said receiving member from said first image support member; and

(e) applying electroscopic marking material to the free surface of said receiving member subsequent to contacting said second image support member but prior to separation of said receiving member therefrom, thereby simultaneously forming a plurality of visible powder images on said receiving member.

10. The method as defined in claim 9 and further including the step of fixing said plurality of visible powder images on said receiving member.

11. The method as defined in claim 9 wherein said receiving member comprises a material having a bulk resistivity of from about 10 to 10 ohm-cm.

12. The method as defined in claim 9 wherein said receiving member is a continuous web and further including the steps of transferring said plurality of visible powder images from said receiving member to a permanent copy sheet and fixing said images thereto.

13. The method as defined in claim 12 wherein said electroscopic marking material is applied to said receiving member by a magnetic brush development system.

14. The method as defined in claim 9 wherein a potential is applied to said receiving member at least in the portions thereof desired to be imaged, said potential being applied subsequent to contacting said receiving member with said first image support member but prior to separation of said receiving member therefrom and wherein said electroscopic marking material is applied by a conductive developer system.

15. The method as defined in claim 14 wherein said conductive development system comprises a conductive magnetic brush developer system.

16. The method as defined in claim 9 wherein the electrostatic charge patterns carried by said first and second imaging members respectively are opposite in image sense but are comprised of electrostatic charges of the same polarity.

17. The method as defined in claim 9 wherein the electrostatic charge patterns carried by said first and second imaging members respectively are the same in image sense but are comprised of electrostatic charges of opposite polarity.

18. The method as defined in claim 9 wherein said electrostatic charge pattern carried by said second image 16 support member is negative in image sense and said electroscopic marking material is applied to said receiving member by a conductive developer system.

19. The method as defined in claim 18 wherein said conductive developer system comprises a conductive magnetic brush development system.

20. The method as defined in claim 9 wherein said receiving member comprises a substantially humidity insensitive material.

21. A method for forming a plurality of visible powder images on a receiver material comprising the steps of:

(a) providing first and second image support members, each of said image support members bearing an electrostatic charge pattern on a surface thereof;

(b) contacting the electrostatic charge pattern bearing surface of said first image support member with a receiving member having a bulk resistivity of from about 10" to about 10 ohm-cm, thereby inducing a first latent electrostatic image in said receiving member;

(c) contacting the electrostatic charge pattern bearing surface of said second image support member with the surface of said receiving member opposite from that contacted by said first image support member thereby inducing a second latent electrostatic image in said receiving member;

(d) separating said receiving member from said first image support member; and

(e) separating said receiving member from said second image support member while applying electroscopic marking material to said receiving member thereby simultaneously forming a plurality of visible powder images on the receiving member.

22. The method as defined in claim 21 and further including the step of fixing said plurality of visible powder images to said receiving member.

23. The method as defined in claim 21 wherein said receiving member is a continuous web and further including the steps of transferring said plurality of visible powder images from said receiving member to a permanent copy support and fixing said images thereto.

24. The method as defined in claim 23 wherein said receiving member comprises a material having a bulk resistivity of from about 10 to 10 ohm-cm.

25. The method as defined in claim 23 wherein said electroscopic marking material is applied to said receiving member by a magnetic brush development system.

26. The method as defined in claim 21 wherein a potential is applied to said receiving member at least in the portions thereof desired to be imaged, said potential being applied subsequent to contacting said receiving member with said first image support member but prior to separation of said receiving member therefrom and wherein said electroscopic marking material is applied by a conductive developer system.

27. The method as defined in claim 26 wherein said conductive developer system comprises a magnetic brush developer system.

28. The method as defined in claim 21 wherein the electrostatic charge patterns carried by said first and second imaging members respectively are the same in image sense and are comprised of electrostatic charges of the same polarity.

29. The method as defined in claim 21 wherein the electrostatic charge patterns carried by said first and second imaging members respectively are opposite in image sense and are comprised of electrostatic charges of opposite polarity.

30. The method as defined in claim 21 wherein said receiving member comprises a substantially humidity insensitive material.

(References on following page) 7 18 References Cited 3,322,538 5/1967 Redington 961.1 T P T NT 3,332,396 7/1967 Glundlach 118-637 23 A E S 117 17 5 3,545,969 12/1970 Herrich et a1. 96-1 R d1 3/1959 Mayer 117 175 5 3 551146 12/1970 Gun M11! 96 IR 1( garlson etlaL l WILLIAM D. MAR'IIIN, Primary Examiner 1'. 4/1963 M. SOFOCLEOUS, Asslstant Examlner 9/1964 Schaffert 96-1 R 9/1966 Robinson 96-1 R 10 10/1966 Kaiser 96-1 R 961 R, S, D; 118-637 11/1966 Lehmann 961 R 

